Steel strip of thicknesses less than 3 mm and widths of 600 mm to 2200 mm is one of the main products of the steel industry. Approximately two-thirds of this product is normally thinner than 1.2 mm, and this thin strip is a staple of the packaging, appliance, electronics, and automobile industries. Such thin strip can normally only be made to close tolerances, both physically and metallurgically, by cold rolling.
When an attempt is made to hot roll steel to a thickness of between 0.5 mm to 2.0 mm, or more particularly to 0.7 mm to 1.5 mm, it is normally impossible to produce a uniform cross section in the product, and to make it planar. Typically the thinnest strip that can be produced to usable tolerances is between 1.5 mm and 2.0 mm thick. Normally in widths of 600 mm to 1200 mm it is possible to make a usable product 1.8 mm thick, and even to work accurately on a regular basis to 1.5 mm thick between 600 mm and 800 mm wide. Even when subsequent cold rolling, finishing, and heating is used to improve the product, it is normally impossible to hot-roll a thinner workpiece. For instance up to six rolls have been used in a hot-rolling line to produce a workpiece between 1.0 mm and 1.2 mm thick and more than 1000 mm wide, but the results are normally of poor quality.
The problem with hot-rolling procedures is that the strip is very hard to control at the downstream end. The edges fold over or become wrinkled. Even when the workpiece is leveled by stretch bending before the last roll stand or before coiling it up, it is difficult to control it in the output stages. As the workpiece thickness decreases to the common 0.7 mm to 1.2 mm range and the width increases to the common 2200 mm level, the problems multiply.